Optical heads, whether of the bulk or integrated optic variety, must provide at least three separate functions. First of all, it must retrieve the data signal encoded on the light reflected off of the optical disk. Second, it must measure tracking errors that are displacement errors associated with motion of the disk in the plane of the disk. Finally, it must measure focus errors that are displacement errors associated with motion of the disk in a direction perpendicular to the plane of the disk. Integrated Guided Wave Optical Heads (IGWOH) represent compact, low mass alternatives to bulk optical head assemblies for reading and writing information on optical data storage disks.
U.S. Pat. No. 4,798,437 discloses a method and apparatus for processing analog optical wave signals. This patent describes the use of Mach-Zehnder (M-Z) interferometer arrays formed on inorganic electro-optic substrates, such as LiNbO.sub.3 for example, to analyze wavefront profiles. Among the devices mentioned is an integrated optical waveguide range finder wherein free space radiation from a point source is end-fire coupled into an array of channel waveguides. The distance of the point source from the analyzer determines the curvature of the wavefront incident on the analyzer. Portions of the wavefront falling on adjacent channel waveguides are shifted slightly in phase due to this curvature. Adjacent channels are joined together at a Y-junction and the relative phase shift results in interference between light from these two channels. The signal exiting the interferometer, when taken together with the signals from other interferometers in the array, provide a measure of the wavefront curvature and hence of the distance to the source. Other light signals provide intensity reference levels. Control electrodes permit electro-optical phase-shifting of one channel with respect to the other. This feature provides a means of compensating for slight differences in the optical path lengths of the two channels or of biasing one channel with respect to the other.
It is desirable to use newer technology and have waveguides formed from new thin-film electro-optic polymers deposited on silicon substrates and to integrate electronic functions such as photodetection, amplification, and switching with the optic functions. Current conventional bulk heads for readout of optical data are limited in performance by weight and physical size. Integrated optic heads offer the possibility of low weight, compact size, and higher performance. A co-filed application entitled Waveguide Optical Pick-Up Head Using a Mach-Zehnder Interferometer Wavefront Sensor, which is incorporated herein by reference, descries an integrated optic waveguide pick-up head which can be more easily fabricated than prior art integrated optic heads. It is necessary to impose a .+-.90.degree. phase retardation on one arm relative to the other of each channel waveguide Mach-Zehnder (M-Z) interferometer in order for the device to function properly. A non-linear optical material exhibiting the so-called linear electro-optic or Pockels effect is incorporated in or deposited on top of one arm of each M-Z interferometer and permits voltage controlled tuning of the phase retardation. Inorganics such as LiNbO.sub.3 and organics such as spin-coated azo-benzene sulfone dye co-polymer are examples of suitable non-linear optic materials.
The idea of using M-Z interferometers to sense focus error in a waveguide optical pick-up head was advanced in the co-filed application. The use of the Langmuir-Blodgett (L-B) technique as a means of forming optically transparent poled films of noncentrosymmetric molecules was advanced by Penner, et al., in co-pending application Ser. No. 07/735,550, filed Jul. 25, 1991 entitled Improved Conversion Efficiency Second Harmonic Generator. It is desirable to extend the use of L-B films to include use as the nonlinear optical medium in one arm of M-Z interferometers used in waveguide optical pick-up heads.